Much attention is drawn to organic light-emitting devices such as organic electroluminescence (EL) devices usable for surface-emitting devices. Promising as inexpensive, solid-emission-type, large-emission-area, full-color display devices and writing light source arrays, the organic light-emitting devices have been actively developed. The organic light-emitting device generally comprises a couple of electrodes (a transparent electrode and a rear-surface electrode), and a light-emitting, organic, thin-film layer formed between the electrodes. When an electric field is applied to the organic light-emitting device, electrons are injected into the light-emitting, organic, thin-film layer from the rear-surface electrode, while holes are injected thereinto from the transparent electrode. Electrons and holes are recombined in the light-emitting, organic, thin-film layer, and an energy level is lowered from a conduction band to a valence band, whereby energy is turned to light, which is emitted from the organic light-emitting device.
The organic, thin-film layers in the organic light-emitting device are generally formed by a vapor deposition method. For instance, JP 9-167684 A and JP 2000-195665 A propose methods comprising uniformly forming an organic layer on a temporary support of mica or a film by a vapor deposition method, bringing the organic layer close to the substrate, and carrying out a heating vapor deposition. However, these methods are poor in productivity because they use a vapor deposition method. In addition, because only low-molecular-weight organic compounds can be used for organic, thin-film layers, the resultant organic light-emitting devices are insufficient in durability such as bending resistance, film strength, etc. when used for flexible displays, etc. This problem is serious particularly when they have large areas.
In view of the above problems in connection with the use of low-molecular-weight organic compounds, proposals have been made to provide high-molecular-weight organic EL devices comprising light-emitting thin-film layers made of high-molecular-weight compounds, and those comprising light-emitting thin-film layers constituted by low-molecular-weight compounds dispersed in binder resins. For instance, Nature, Vol. 347, page 539, 1990 proposes a green-light-emitting, high-molecular-weight, organic EL device using poly(p-phenylenevinylene). The Japanese Journal of Applied Physics, Vol. 30, page L1938, 1991 proposes a high-molecular-weight, organic EL device using poly(3-alkylthiophene) for emitting red orange light. The Japanese Journal of Applied Physics, Vol. 30, page L1941, 1991 proposes a blue-light-emitting, high-molecular-weight, organic EL device using polyalkylfluorene. These high-molecular-weight devices are advantageous in making large-area, light-emitting devices, and their applications for flexible displays are expected. However, because the vapor deposition method cannot be used to form the organic light-emitting thin-film layers, thin-film layers are formed directly on substrates by wet methods.
The wet methods are, however, disadvantageous in that the formed organic, thin-film layers are insufficient in the uniformity of thickness because of the surface tension of solutions, and that when the organic, thin-film layers are laminated, the organic, thin-film layers tend to be dissolved in their interfaces. Accordingly, the organic, thin-film devices obtained by the wet methods are poor in light-emitting efficiency and durability of devices.
WO 00/41893 discloses a method for thermally transferring an organic, thin-film layer and a photo-thermal conversion layer onto a substrate by a laser beam by using a donor sheet having the organic, thin-film layer and the photo-thermal conversion layer. Such a thermal transfer method is disadvantageous in that a gas often penetrates into an interface between the organic, thin-film layer and the substrate. In the organic light-emitting device produced by such method, light-emitting efficiency, durability and uniformity of a light-emitting surface depend on conditions of the interface, the penetration of gas resulting in poor light-emitting properties.
In the case of thermal writing in a predetermined pattern using a thermal head or a laser common in printing technologies, a temperature distribution generated around an organic thin film pattern by thermal diffusion blurs its outline, failing to cut the organic thin film pattern from the donor accurately. Thus, organic light-emitting devices produced by this method are uneven in light emission and likely to suffer from poor durability, because of insufficient electric connection and the breakage of the organic, thin-film layer. Further, yield is likely to be low because of low-accuracy positioning of the substrate and the thermal head or laser beam.